On Some Common Errors in Iron Bridge Design

18 ment. First, the diagonal tension, which is approximately equal to the diagonal compression, has a powerful tendency to prevent buckling or undulation and so improve matters, and second, to apply a safety factor of six to a long column, whose failure is clue to lack of stability and not of strength is unscientific as is shown in my paper on this subject in the “ Transactions of the Royal Society of Victoria.” Vol. XV., p. 14. There is no doubt that the resistance of such a web to buckling is at least five times as great as the Rankine treatment will allow, and this conclusion is confirmed by Professor Warren’s elaborate analysis of the web stresses of Penrith bridge, in the Royal Commission Report previously quoted. It is there shown that, according to Rankine, the safety factor of some parts of the web is less than unity under ordinary traffic, and yet this bridge has now been in existence for more than thirty years and has shown no sign of weakness. To determine the exact strength of a thin plate web against buckling is a question of much difficulty and obscurity, but there is no possible doubt that it is immensely greater than Rankine’s imperfect method of computing indicates, and that in a vast number of instances the thinnest metal that it is desirable to use from the point of view of corrosion, and practical convenience of construction is abundant to resist the tendency to buckle clue to the shear. The following rules may be laid down as sound for arranging verticals, and proportioning webs. 1. A vertical pillar at each point of support of section pro- portioned to the reaction of that support should extend from bottom to top of the web, but it needs the full section, only at the bottom and may taper to nothing at the top. 2. This pillar should be placed fairly on the centre of the support and not as is sometimes seen at or near one edge. 3. The web should have a vertical sectional area of one square inch for not more than 2 tons of vertical shear for wrought iron, and 2| tons for mild steel. 4. Vertical stiffeners of T section should be placed wherever any considerable concentrated load is imposed on the top chord.